The first tip is to limit your library for a bit of time as students pick their books. My students pick new books for their boxes once a week. They pick every Friday. I have about twenty different totes of books in my class library. They are all divided and labeled clearly. At the beginning of the year, I tell the students about the contents of each box. However, I do not allow students to pick out of all twenty totes in the beginning. It would be just too overwhelming.
Instead, I usually put out about six totes for students to choose from. I allow them to pick six books. Over the next weeks I add one tote a week until all the totes have been introduced. Limiting the number of the totes helps the book selection process keep moving along.
As far as teaching students to pick appropriate books, I love the lesson presented by the authors of The Daily Five. However, I found another little resource. BrainPOP Jr. has a video lesson about picking "good fit books." This video reinforces all the points you will want to impress upon students when picking books: interest level, purpose, readability, etc.
To be honest with you, picking appropriate, on level books is a challenge for young students. I review my Good Fit books anchor chart every week before students beginning picking books. The BrainPOP video is also a great quick little review. Continuing to review and discuss what makes a book a good fit will eventually help students make quality choices.